Support constref parameters passing.

[csyonghe]

This PR adds support for __constref parameter modifier to specify that a parameter should be passed by const reference.
A constref parameter is treated as an immutable value.
Adds [constref] attribute that can be used to mark a function to make this parameter passed by const reference.

Make NonCopyable type passed by constref by default.
Also makes sure constref parameters doesn't lead to redundant load/stores in auto-diffed functions.

Add tests for mixing use of __constref and no_diff in a differentiable function.

[saipraveenb25]

Looks good!

Just to clarify, constref isn't going to mix with differentiable types?

[csyonghe]

I haven't tested, but it is likely not going to work to have a differentiable constref parameter. We need to handle that carefully if we decide to support it.

[csyonghe]

One way to support it is to convert it to a normal in parameter.

[csyonghe]

OK, I added checks to prevent forming a constref differentiable parameter, including this and normal parameters.